B1115 – Electric Water Pump Malfunction

DTC B1115 is a Mitsubishi-specific fault code stored by the Heater Controller Assembly (A/C-ECU) when the electric heater water pump fails to run for a continuous 30-second window. Found primarily on the Mitsubishi Outlander PHEV across multiple model years, this code almost always presents as a complete loss of cabin heating in EV mode — often leaving drivers cold while the petrol engine remains seemingly unaffected. While it won’t disable the drivetrain outright, ignoring B1115 can lead to secondary damage including overheating of the PTC heater element and accelerated pump failure.

What Does B1115 Mean?

The Mitsubishi Outlander PHEV uses a dedicated small electric water pump — separate from the engine coolant circuit — to circulate heated coolant through the cabin heater matrix whenever the combustion engine is not running. This pump is commanded on and off by the A/C-ECU (Heater Controller Assembly) via a relay. The ECU monitors pump operation using a speed feedback signal on the EWPS line. When the pump fails to reach the expected operating state within 30 seconds of being commanded on, the A/C-ECU stores B1115 as a stored (past or current) fault and may extinguish the heater request entirely to protect the PTC heater unit from running without coolant flow.The code covers three root scenarios: the pump motor itself has failed mechanically or electrically; the pump relay or supply wiring is open-circuit; or a blocked inline coolant filter is preventing flow and causing the pump to stall under load. All three result in the same logged fault — the A/C-ECU cannot distinguish between them without further pinpoint testing.

Symptoms

• No cabin heat in EV mode — blower runs but air remains cold or ambient temperature
• Cabin heating works only when the petrol engine is running
• MUT-III or compatible scanner shows B1115 stored in the A/C-ECU memory
• EV System Service Required warning light on the instrument cluster (in some cases)
• Unusual whining, grinding, or no audible sound at all from the small firewall-mounted pump on startup
• Heater performance that worked briefly after the coolant inline filter was disturbed then failed again
• Other related codes present alongside B1115, such as B1116 (Electric Water Pump Harness) or heater system fault codes

Common Causes

• Blocked inline coolant filter (gauze strainer): A fine-mesh filter sits inside the heater hose above or upstream of the pump. Sediment and scale accumulation — especially from incorrect coolant use — can restrict flow enough to stall the pump motor under load, triggering B1115.
• Failed heater water pump assembly: The small 12V pump mounted on the firewall (passenger side) is a known wear item on the Outlander PHEV. Internal motor failure, seized impeller, or bearing wear prevents operation entirely.
• Open circuit in pump relay or supply wiring: The pump is powered through a dedicated relay. A failed relay, blown fuse, or open circuit on the EWPB power supply line will prevent the pump from receiving voltage regardless of A/C-ECU commands.
• Damaged or corroded wiring harness: Water ingress into connector bodies, particularly at the pump connector and the heater water pump relay block, causes high resistance or open circuits on the power, ground, or EWPS feedback lines.
• Faulty heater water pump relay: The relay itself can fail in the open position, cutting power to the pump without triggering a harness fault code.
• A/C-ECU fault: In rare cases, the Heater Controller Assembly fails to command the relay on, resulting in no pump activation signal. This is a last-resort diagnosis after all wiring and pump checks are exhausted.
• Air lock in the coolant circuit: Air trapped in the small heater loop after coolant service can cause the pump to cavitate and fail to generate sufficient flow, leading the ECU to interpret this as a pump malfunction.

Common Misdiagnoses

• Replacing the pump without inspecting the inline filter first: The filter blockage is the most common — and cheapest — fix. Technicians unfamiliar with the PHEV heater circuit often go straight to pump replacement, only for the new pump to fail again if the filter remains blocked.
• Blaming the PTC heater or high-voltage system: Because EV-mode heating is absent, technicians sometimes assume the high-voltage PTC heater element is at fault. B1115 is set by the A/C-ECU and relates to the 12V coolant pump circuit — not the HV heater element itself.
• Overlooking the heater water pump relay: The relay is a frequently missed component. It’s low-cost and easy to swap, but often skipped when technicians focus directly on the pump assembly.
• Assuming air lock has resolved after bleeding: A partial bleed may temporarily restore pump operation before trapped air migrates back into the pump inlet, causing intermittent faults and recurring B1115 codes.
• Attributing the fault to the main 12V auxiliary battery: Low auxiliary battery voltage can suppress heater system operation and store multiple codes including B1115. Always verify 12V battery state of health before condemning heater system components.

Affected Vehicles

Make	Model	Years	Notes
Mitsubishi	Outlander PHEV (GG2W / GG3W)	2013–2021	Primary affected platform; all trim levels with cabin electric heater
Mitsubishi	Outlander PHEV (GN0W)	2022–present	Revised PHEV platform; same code family applies to heater pump circuit
Mitsubishi	Eclipse Cross PHEV	2021–present	Shares similar A/C-ECU heater pump architecture; B1115 applicable

Tools & Equipment

Tool	Purpose	Notes
Mitsubishi MUT-III or PHEV-capable OBD2 scanner	Read/clear B1115; run actuator test No.23 (Electric Water Pump)	Generic OBD2 scanners may not access A/C-ECU B-codes
Digital multimeter (DMM)	Voltage and continuity checks on relay, supply, ground, and EWPS lines	Essential for wiring pinpoint tests
Relay test kit or known-good spare relay	Swap-test the heater water pump relay	Relay part number varies by model year — confirm before purchasing
Coolant drain pan and fresh coolant	Drain and refill heater loop when removing pump or filter	Use Mitsubishi-approved PHEV coolant to prevent filter re-blocking
Hose clamp pliers	Release spring clamps on pump inlet/outlet hoses	Access is tight on GG2W — remove any blocking brackets first
Flashlight / inspection mirror	Locate inline filter and assess connector condition	Filter is inside the hose above the pump — easy to overlook

Step-by-Step Diagnosis

• 1
  Verify the fault and check freeze frame dataConnect your scanner to the OBD2 port and access the A/C-ECU (Heater Controller Assembly) module. Confirm B1115 is present. Note any additional codes — B1116 (pump harness) stored alongside B1115 points strongly to a wiring fault rather than a dead pump motor. Check freeze frame conditions to identify whether the fault sets at startup or during operation.
• 2
  Check auxiliary 12V battery voltageMeasure the auxiliary (12V) battery voltage with a DMM. It must be at or above 12.4V at rest. A weak auxiliary battery can cause the heater system to fail to initialise and store phantom B1115 codes. If battery voltage is below spec, charge or replace the battery and retest before proceeding.
• 3
  Check coolant level and inspect for air locksVerify coolant level in the reservoir is between MIN and MAX. A low level introduces air into the heater pump circuit, causing cavitation and stall. If the level is low, top up with manufacturer-approved coolant, bleed the heater circuit, then clear B1115 and retest using the actuator test.
• 4
  Run Actuator Test No.23 — Electric Water PumpUsing MUT-III or an equivalent capable scanner, navigate to the A/C-ECU actuator test and command the electric water pump on (Test No.23). Listen and feel at the pump body for vibration/operation. If the pump runs during the actuator test but B1115 returns under normal operation, suspect an intermittent fault — likely a blocked filter causing periodic stalling under load.
• 5
  Inspect and clean the inline coolant filter (gauze strainer)Locate the fine-mesh gauze strainer inside the heater inlet hose above the pump. Remove the hose, extract the filter, and inspect for sediment, scale, or debris blockage. Clean thoroughly or replace. This is the single most common cause of recurring B1115 on the Outlander PHEV and should never be skipped.
• 6
  Test the heater water pump relayLocate the heater water pump relay in the relay block (refer to your model year’s service manual for exact position). Remove and swap with a known-good relay of the same type, or use a relay tester. Measure voltage at relay terminal 4 and terminal 2 with the ignition on — both should show battery voltage on the supply side. A relay that fails open will cut all power to the pump.
• 7
  Inspect and test the wiring harnessWith the relay removed, check for open or short circuits on the power supply lines between the fusible link and relay connector terminals. With the pump connector disconnected, check for open or short circuits on the EWPB (pump power) and EWPS (pump speed feedback) lines between the pump connector and the A/C-ECU connector. Repair any damaged or corroded wiring before condemning the pump.
• 8
  Test the pump assembly directlyWith wiring confirmed good, apply 12V directly to the pump terminals (observe polarity). The pump should spin freely and audibly. If it does not operate, is seized, or draws excessive current, the pump assembly has failed and requires replacement.

Scanner Readout Explained

Module:    A/C-ECU (Heater Controller Assembly)
DTC:       B1115
Status:    Stored / Past Fault
Condition: Heater water pump assembly did not operate for 30 seconds
           after being commanded ON by A/C-ECU

Related codes to check:
  B1116 — Electric Water Pump Harness (open/short on EWPB or EWPS lines)
  B1108–B1114 — Electric water heater sensor faults (PTC circuit)
  U1073 — BUS-OFF (CAN communication loss — check 12V battery first)

Actuator Test:
  No.23 — Electric Water Pump
  Result: PASS = pump runs normally during commanded test
           (suspect intermittent / filter blockage)
  Result: FAIL = no vibration/sound at pump
           (relay, wiring, or pump assembly fault)

Key data PIDs to monitor:
  Heater water pump command: ON/OFF
  Heater water pump status: Operating / Not Operating
  Coolant inlet temp sensor (B1111/B1112 range)
  Coolant outlet temp sensor (B1113/B1114 range)

Step-by-Step Repair Guide

Repair Path A — Clean or Replace the Inline Coolant Filter

• 1
  Locate the inline filterThe gauze strainer is seated inside the heater inlet hose immediately above and upstream of the electric water pump on the firewall. On GG2W models it is accessible from the engine bay without removing major components, though some brackets may need to be moved aside.
• 2
  Drain the heater loop coolantPlace a drain pan below the pump. Release the spring hose clamp on the inlet hose and lower it carefully to drain coolant from the heater circuit. Avoid allowing coolant to contact paintwork or electrical connectors.
• 3
  Remove and inspect the filterExtract the gauze strainer from the hose end. Inspect for sediment, rust, or scale. If partially blocked, flush clean with water. If heavily blocked or physically damaged, replace with Mitsubishi OEM part (refer to model year parts catalogue). On early-production 2013–2014 models, a revised filter design was released — confirm your replacement matches the current part number.
• 4
  Flush the heater circuit and refillBefore refitting, consider flushing the heater matrix circuit to remove any remaining sediment. Refill with fresh Mitsubishi-specified PHEV coolant (do not use generic antifreeze — it is associated with faster sediment formation in this system). Bleed air from the circuit thoroughly by pinching hoses repeatedly with the system running.
• 5
  Clear B1115 and verifyClear the fault with your scanner, run Actuator Test No.23 to confirm the pump operates, then verify cabin heating in EV mode over a full drive cycle. Confirm B1115 does not return.

Repair Path B — Replace the Heater Water Pump Assembly

• 1
  Confirm pump failureVerify through direct 12V bench test or actuator test failure that the pump assembly itself is faulty. Ensure the relay, fuse, and wiring are confirmed good before ordering a replacement pump — this avoids unnecessary parts cost.
• 2
  Disconnect the electrical connector and drain coolantDisconnect the pump’s electrical connector. Drain the heater loop coolant into a catch pan by releasing the inlet hose clamp.
• 3
  Remove the pump mountingRelease both inlet and outlet hose clamps from the pump. Undo the pump mounting bracket bolts (typically two or three M8 bolts). Remove the pump assembly from the firewall. Inspect the mounting location for corrosion or bracket damage.
• 4
  Install new pump and replace filter simultaneouslyFit the new pump assembly. This is also the ideal opportunity to fit a new inline filter, regardless of its apparent condition. Reconnect hoses and tighten clamps. Reconnect the electrical connector and verify it locks fully.
• 5
  Refill, bleed, and verifyRefill the heater circuit with fresh OEM-specified coolant. Bleed air thoroughly. Clear B1115, run Actuator Test No.23 to confirm pump operation, then validate cabin heating in EV mode over a drive cycle.

Repair Path C — Repair Wiring or Replace Relay

• 1
  Replace the heater water pump relayRemove the identified relay from the relay block. Install a confirmed replacement of the correct type. Retest pump operation with the actuator test. If the pump now operates, the relay was the fault. Clear B1115 and confirm over a drive cycle.
• 2
  Repair identified harness faultsIf multimeter testing identified open or high-resistance circuits on the EWPB (pump power) or EWPS (pump speed signal) lines, repair the affected wiring. Clean connector bodies with electrical contact cleaner and apply dielectric grease to prevent future corrosion ingress. Replace connector pins that are pushed back, bent, or corroded beyond cleaning.
• 3
  Clear codes and retestAfter all wiring repairs, clear all A/C-ECU codes and perform a full drive cycle in EV mode to confirm B1115 does not return. Re-scan all modules to check for any secondary codes that may have been masked by the original pump fault.

Repair Cost Estimates

Repair	DIY Cost	Workshop Cost	Notes
Inline coolant filter clean/replace	£5–£20 / $5–$25	£60–£120 / $70–$140	Most cost-effective first step; often resolves the fault entirely
Heater water pump relay replacement	£10–£25 / $12–$30	£50–£100 / $60–$120	Low-cost part; confirm correct relay type for your model year
Heater water pump assembly replacement	£80–£180 / $95–$210	£250–£450 / $290–$520	OEM pump recommended; aftermarket availability is limited
Wiring harness repair (pump circuit)	£15–£50 / $18–$60	£150–£350 / $175–$400	Highly variable; corrosion repairs can be labour-intensive
A/C-ECU replacement (last resort)	£200–£600 / $235–$700	£500–£1,200 / $580–$1,400	Rarely required; exhaust all other causes before proceeding

Prevention & Maintenance Tips

• Use only Mitsubishi-specified PHEV coolant in the heater circuit. Generic antifreeze formulations are associated with accelerated scale and sediment formation, which is the primary cause of inline filter blockage and repeat B1115 faults.
• Inspect and clean the inline gauze filter at every major coolant service interval or any time the heater hoses are disturbed — this takes minutes and prevents the most common cause of pump failure.
• Bleed the heater circuit thoroughly after any coolant work. Air locks cause the pump to cavitate and can trigger B1115 even on a fully serviceable pump.
• Check the auxiliary 12V battery condition annually. A weak aux battery causes voltage drop events that stress the heater system electronics and can store phantom codes across the A/C-ECU and other modules.
• Monitor for early symptoms — reduced heating in EV mode before a full failure gives advance warning of a partially blocked filter or a pump beginning to slow. Acting early avoids the fault progressing to B1115.
• Inspect the pump electrical connector for water ingress and corrosion during each coolant service. Apply fresh dielectric grease to the connector body seal to prevent moisture from tracking into the terminals.

Frequently Asked Questions

Will B1115 cause the Mitsubishi Outlander PHEV to stop driving?

In most cases, no. B1115 is a heater system fault and will not directly prevent the PHEV from driving. However, if B1115 appears alongside a broader EV system fault (such as a high-voltage pre-charge failure), the vehicle may enter a limp or no-drive condition. If the car refuses to engage drive with B1115 present, scan all modules and address any accompanying codes before focusing solely on the pump circuit.

Can I drive with B1115 stored?

You can drive the vehicle, but cabin heating in EV mode will be absent or severely reduced. The more important concern is protecting the PTC heater element — avoid activating the cabin heater if you know the pump is not running, as operating the PTC without coolant flow risks overheating and damaging a component that costs considerably more than the pump itself.

Why does my Outlander PHEV only heat up when the petrol engine is running?

This is the classic symptom of B1115. When the combustion engine runs, coolant from the engine circuit flows through the heater matrix via a different path — the electric water pump is only needed to circulate coolant through the cabin heater when the engine is off. A failed pump therefore produces exactly this result: warmth when the engine runs, cold air in EV mode.

How do I know if it’s the filter or the pump that has failed?

Run the actuator test (No.23 on MUT-III or equivalent). If the pump operates normally during the test but B1115 returns under real driving conditions — especially after the system has been running for a few minutes — a partially blocked filter is the likely culprit. If the pump fails to run or vibrate at all during the actuator test even with good relay and wiring supply voltage, the pump assembly itself has failed.

Can a weak 12V auxiliary battery cause B1115?

Yes. A degraded auxiliary battery that drops voltage during heater system initialisation can prevent the pump relay from operating correctly, causing the A/C-ECU to log B1115 even when the pump and wiring are physically sound. Always check and confirm auxiliary battery health as an early step in the diagnostic process — it is one of the most frequently overlooked causes of multi-code heater system faults on the Outlander PHEV.

What coolant should I use in the Mitsubishi Outlander PHEV heater circuit?

Mitsubishi specifies their own PHEV-approved coolant for this circuit. Using a generic or incompatible antifreeze is widely reported by PHEV owners and technicians as a contributing factor to rapid filter blockage through scale and sediment formation. Always use the correct specification coolant, and replace coolant at the manufacturer’s specified intervals rather than extending service periods.

Is B1115 covered under any Mitsubishi warranty or extended warranty?

Coverage depends on your vehicle’s age, mileage, and the warranty terms in your region. In some markets, early Outlander PHEV heater system failures have been addressed through dealer goodwill or extended warranty programmes — particularly for vehicles within a certain age or mileage threshold. It is always worth contacting your Mitsubishi dealer to discuss coverage before proceeding with out-of-pocket repairs, especially if the pump failure is not related to coolant contamination from incorrect servicing.

Will clearing B1115 fix the problem?

Clearing the code without addressing the root cause will only result in B1115 returning within one drive cycle — or immediately if the fault is current rather than intermittent. Clearing codes is a useful step for confirming a repair, but it is not a fix in itself. Always resolve the underlying pump, filter, relay, or wiring fault before clearing and retesting.

Conclusion

DTC B1115 on the Mitsubishi Outlander PHEV is a well-documented heater circuit fault that leaves drivers without cabin heat in EV mode while often appearing deceptively minor. The diagnostic path is logical and well-defined: start with the auxiliary battery and coolant level, run the actuator test, inspect the inline filter (the most common culprit), and work methodically through the relay and wiring before condemning the pump assembly itself. A blocked gauze filter costs almost nothing to clean and resolves a significant proportion of B1115 cases entirely — making it the essential first step that should never be skipped. Where pump replacement is genuinely required, pairing it with a fresh filter and an approved coolant refill gives the best protection against a repeat fault.